US2284444A - Demodulation circuit - Google Patents

Description

May 26,1942. E. PETRsoN 2,284,444

DEMODULATION CIRCUITS Filed Aug. 27, 1940 L Osc 4 l LIM/TER c, L, En /l/ /2 HNL Moa "E 4M? 7??. CJ AEE me Re: /7 f \a 5 6 9 R3 F IG. 5

NoN-WEAR I I I mmm/c6 d \/d bw PETERSON A TTOR/VEV Patented May 26, 1942 UNlT-ED STATES PTENT, OFFICE I DEMODULATION CIRCUIT Eugene Peterson, New York, N.' Y., assignolrto Bell Telephone Laboratories, Incorporated,V .New York, N. Y., acorporation of New York Application August 27, 1940, Serial No. 354,351

(Cl. Z50- 20) 9 Claims.

This invention relates to detection or demodulation of wave-length modulated waves, that is, frequency modulated or phase modulated waves, in high frequency signals and more particularly to'circuit arrangements yin which use is made of the properties of saturated core-inductances of the type described in'patent to Wrathall 2,117,752, May 17,1938. n

The purpose 'of the invention is to provide a means for securing a frequency indication or a phase indication and Vto this' end I provide means whereby pulsesare'generated, the frequencv of which is determined solely by thev momentary frequency for frequency modulated waves,'or the time displacements of which from normal or vno signal condition is proportional to vthe momentary phase displacement in phase modulating waves. In order at the same time to render the received signal dependent solely on frequency or on phase modulation, use is made of amplitude limiting devices all in a manner hereinafter described. Y

The invention will be better understood by reference to the followingspeciiication and the ac-l companying drawing, in' which:

Fig. 1 isa diagram of a circuit adapted for demodulation or detection of aY frequency modulated wave;

Fig. 2 discloses a modification of a part of the circuit of Fig; 1;

Fig. 3 shows curves for explaining 'the principles involved inthe circuit of Fig. 1;

Figs. 4 to 6 are modifications rof a detail of Fig. 1;

Fig. 7 shows a circuit adapted for detection or demodulation of a phase modulated wave;

Figs. 8 and 9 are curves for explaining the principles involved in the circuit 'of Fig. 6; and

nected 'to themodulator to passthis intermediateV frequency-and itsside-b'ands, and Vthe output of `the vband-pass lter is then impressed on a circuit containing the non-linear magnetic coil 1. This circuit is ofv the..type described inthe patent to Wrathall referred kto above and possesses the property of generating sharp impulses of very short duration with onepulse for each positive loop and one .in the reversedirection for each negative loop of the impressed wave. This is. illustrated in Fig. 3 which shows schematically a short section of'a frequency modulated wave, The generatedimpulses, also shown in this figure, occur very nearly at the timewhen the current wave inv the coil is passing through itszero value. It is apparent that were one to integrate the effect of all of these pulses the resultwould be substantially Zero. In accordance with one form of myV invention, however, I connect across the coil 1 a rectifier element 9 which suppresses the impulses of the one sign and the` g integrated value of the Vremainder willbe rep- Fig. l0 illustrates the generation' of control waves and transmission thereof to atran'smitting and a receiving station.

Referring more particularlyto Fig. 1, there is shown an incoming circuit for a received frequency modulated wave, this incoming circuit being an antenna system, as shown, or an appropriate line circuit. The incoming wave Vmay be impressed directly on a harmonic generator coil I or it may be first impressed on a modulator circuit 3 vto which is also connected an oscillator 4 of controlled frequency, the frequency resented by a low frequency current in a translating device l'l, the amplitude of the current being proportional to, the Ynumber of pulses generated per .unit of time and thus proportional to the momentary frequency of the original incom- In another form `of my invention I use .bothpulses by the full'wave rectifier of Fig. 2. Obviously the output voltage across the resistance R3 will contain high frequency components. These may be filtered out so far as the translating device I7 is concerned by a low-pass nlter Il. Appropriate amplifiers, such as 6 and I2, maybe inserted as desired. l A In a frequency modulated wave it is desirable that there shall be no amplitude modulation present. Due to various imperfections, however, in the circuitfor in thev transmitting medium, variations in amplitude may enter. These effects largely disappear in using coil 'I but not entirely so,.for itis found that the amplitude of the peaks generated by the coil is substantially proportional to the four-tenths power of the input.

In order to more fully. eliminate the corresponding disturbances I find it desirable in some instances to associate withthe circuit some amplitude limitingdevice such` as that shown at I4. While such a limiting device may be introduced at anyof various points in the circuit, I have shown it as bridged across the condenser C3. In

this case the limiting device may take-on a variety of forms such as shown in Figs.,4 to 6. For example, in Fig. 4 the limiting device is` shown 1S WO diode rectifiers which will substantially prevent the voltage across condenser C3 from rising above a specified value. In Fig. 5 the limiting device is shown as a thyrite resistor, and in Fig. 6 it is shown as two oppositely directed copper-oxide rectiers connectedA in parallel and j biased by suitable voltages so thatV they serve as` virtually open circuits until the voltage over C3 rises above a specified value. Y Y In one mode of operationY ofmagnetic harmonic generatorsthe condenser C3 is made very large and R3 is made so largethat it is virtually an open circuit, in which case practically no v'oltage is built up across the condenser and practically no current flows through R3 so that voltage across R3 is substantially .thesame as that across the non-linear coil L. Thisarrangement, which may be spoken of asv open vcircuitopera tion, is found particularly advantageous if the an unmodulatedarriving wave occur at the time when the locally generated wave as applied to demodulator I3 is passing through its zero value. Under these conditions the modulation effects, which are vproportional to the instantaneous cross-product of the two inputs, are substantially equal tozero.

If, however, the pulses are dis- Y placedrin time due to phase modulation, then circuit into which the coil works has a high in;

put impedance. For the suppression. of .the pulses of one. sign,'.if such suppression visqused,

therectier maybe placed. in series with R3 or in parallel; fIn eitheryoasve yI findthat the limiter forf the purposes described above isgnotV required, for vifthe instantaneous amplitudel of the signal increases due to such causesasjnoise, the magnetizing force in thecoil rises more rapidly giv ing a hgherreactance voltage butit Vsweeps over the range of high vinductanceof theY coill in a shorter time, that i,s, tl1,eV height ,of the generated peakis somewhat greater .but its duration is. less -so that the area under the peakloop is substantially independent ofthe amplitude of the-sig-y I nal; It is, these areas which are integrated inthe translating device. :Another way to express this isv to note .thatunder these conditions theaveragecurrent through R3 will be the same no matter what the incoming wave amplitude' may be so long as it is above a certaink minimum value. Thus underthese circumstances. the non-linear coiland itsassociated output current functions asaunitary self-limiting detecting device for a frequency modulated wave. [In all cases and .es-

pecially the ,one inwhichthe rectifier isl placed in series with Rz'it. is ydesirable that the rectifier shall bealinear onefor `thatit shall be operated in that part-,forwhich it is substantially linear. j My `circuit'arrangement for detectionor. ciemodulationof aqphase modulated wave .is illus?. trated in Fig. 7. Here again the incomingV phase modulatedwave received over line 2U from distant transmitter I9 is impressedon a harmonic the demodulation cross-product term is proportional to the instantaneous'value of the locally introduced carrier, and thus the amplitude of the resultant low frequency wave is proportional Ato the momentary phase modulation of the incoming wave. In order to provide the precise agreement-between the incoming carrier frequencyfa'nd thatproduced at the receiver some of the wave from source I5 may be supplied over line I8 to the distant transmitter I9 to serve as theV transmitting carrier wave from station I 9.

These facts may be readily shown by thefollowing'w'analysis in which isa-is the' phase dispiacmentatjany m"- This'alon'g'with the locally introduced carrier is impressed onthe demodulating elementso that Inthe outputof thedemodulator theA terms'of interest are primarily the cross-product terms generator 1. bridged. by a rectifier element 9, which latter suppresses impulses of one sign., In

Fig. 8 the impulses are shown in full lines and as .equally spaced, corresponding tothe generation, of the impulses when the Aincoming wave is' notsignal modulated. If, ontheother hand,

Of these, all the alternating current 4cross-product termsare of carrier'frequencies withftheir side-bands or har'mbnics thereof except the one corresponding to n`=1'. This term is the one'low frequency term and appears *as 'A Y J Ilzsin (KQ dos qu Furthermore, if lthe phase displacement is relathe, incoming wave isY phase modulated, then .the

timing of the impulses will be altered forward or balgward fraccordance with .the phase of the modulated..wave. as Yshown by .the dotted impulses.. y

Itis apparent that the mere integration of thesenew. pulses will not'in itself give rise in a translatingdevice toany appreciable signal current., In the circuit.arrangement, however, I impress the pulses on a modulating element I3 on whichthere is also impresseda carrier frequency p/21r from a source I5. This carrier frequency must be vprecisely'that of the incoming carrier frequencyr and is -shown by the sinusoidal curve of Fig, 8. The phase of ythis locally introduced carrier frequency as applied tothe demodulator I3 is` so adjusted that kthe pulses from tively small, that is, if KQ cos qt is small, this then becomes y' @(KQcos qt- I s 2 24 showing that the low frequency outputwisrsubstantially u linear with respect to the original modulating wave, thus giving a good reproduccos Sgt-I4 The case thus treatedisa speciai one but it Y may be generalized. If, .for example, thelocally introduced carrier is some harmonie of the frequency p/21r, such as the kth harmonic, then the input on'the modulator will be given by l [Ebn'cos n pt+ l sin lc pt The only cross-product term which yields signal 2i which may lcontain apparatus similar to that frequency will be that for which n is equal to 7c and this term appears as j A double advantage now becomes apparent. The first of these is the fact that the number of pulses per unit time will be increased by .the factor lc, thus increasing the strength of signal. The second is that the factor corresponding to the depth of modulation, or frequencyl swing, has -`been increased from KQ to kKQ, thatis, has been `increased by' the factor Ic, representing a further increase in signal strength.

It should be apparent' from inspection of .Fig '7 that the range of phase modulation over which the final output is substantially linear with respect to the original signal is that over which the instantaneous value of the locally introduced carrier frequency is substantially linear, that is, in the neighborhood of the cross-over points in the curve of Fig. 8. As sine departs from linearity, the distortion increases. It is possible, however, to increasevery materially the range of linearity by altering the form of the locally introduced carrier frequency. This is illustrated at l,Y 9,- 14 and V|3'of Fig. '7. fA carrier wave of frequency 'p is supplied to station I9` over path Y i8 from a suitable source asinV Figfl. For the in Fig. 9, which makes use of certain disclosures in copending application of Manley-Wrathall Serial No. 354,372, filed August 27, 1940, and consists in introducing in place of a sinusoidal carrier wave, a saw-toothed wave, as shown in Fig. 9. Any suitable method for obtaining such a saw-toothed wave from a sine wave is satisfactory, such as a relaxation circuit, and While rin general it will not be physically possible to duplicate the ideal curve, any step towards this will permit an extension of the amount of phase modulation without appreciable departure from linear output. It should be noted that there would normally have been an impulse corresponding to the points b in Fig. 9, as well as the points a, but any disturbances due to these are eliminated by the rectifier element across the non-linear coil 'I whereby all pulses of one sign are suppressed.

It will be found from the above analysis that the best condition under which a cross-product term of signal frequency is obtained is when the locally introduced carrier frequency is 90A degrees out of phase with the incoming carrier frequency. Thus, since the incoming carrier frequency in the expressions above has been represented by a cosine function, the locally introduced term must be represented by a sine function. This brings out the necessity, as has already been referred to above, of properly phasing the local carrier frequency with respect to the incoming carrier frequency. The importance is also apparent of maintaining the locally generated frequency precisely in step with the incoming carrier frequency. In case my circuits are being used in connection with an extensive wire system, then it is evident that the carrier frequency to be locally introduced may be derived from a source of standard frequency which supplies this standard frequency to the transmitting station also. If such derivation of the locally introduced carrier is not feasible, then it may be obtained by radio transmission from some suitable station of two frequencies, the difference of which is equal to the desired carrier and is obtained by suitable demodulation circuits.

This is illustrated in Fig. 10 in which the transmitting station I9 at line 20 may be as in Fig. 7, leading in this case to a receiving station purpose of supplying a local carrier wave to station 2l of proper vfrequency and properly phased with respect to the incoming carrier frequency, radio transmitter 22 is provided. Thisfsends out two frequencies Fi and F2Y whose difference frequency is equal to the frequency p. 'Ihe two frequencies F1 and F2 are received in radio receiver 26, demodulated at 21 and the difference frequency p is supplied over path I6 to the receiving station 2|. Y

What is claimed is: i `Y l. In a receiving system for wave-length modulated waves, a demodulation circuit comprising a saturable core magnetic harmonic generator adapted to `produce sharp positive and negative pulses, means for impressing thereon wave-length modulated waves of sufficient amplitude to cause saturation of the core of said harmonic generator for a part of each cycle to generate alternate positive and negative pulses timed in accordance with the received wave, means for suppressing pulses of the one sign whereby pulses of one sign remain variably spaced on a time axis in accordance with the wave-length modulation of the received waves, and translating means for producing low frequency output current varying in amplitude in accordance with the variable spacing of said pulses on the time axis.

2. In a demodulation 4circuit for phase modulated waves, a magnetic harmonic generator eX- cted by received phase modulated waves for generation of positive and negative pulses, means for suppressing pulses of one sign, a demodulation element in the circuit on which the remaining pulses are' impressed, and a source of unmodulated carrier frequency associated with the demodulating element, the output of the demodulator for each pulse being substantially proportional to its phase displacement with respect to its own carrier frequency.

3. In a demodulation circuit for phase modulated waves, a magnetic harmonic generator excited by received phase modulated waves for generation of positive and negative pulses, means for suppressing pulses of one sign, a demodulation element in the circuit on which-the remaining pulses are impressed, a source of unmodulated carrier frequency associated with the demodulating element and so phased that there is substantially no demodulation product from pulses due to unmodulated waves but for other pulses the demodulation product is substantially proportional to their phase displacements.

4. The combination of claim 2, characterized by this, that the source of locally introduced unmodulated carrier frequency yields a saw-toothed wave whereby the demodulation output is linear with respect to phase displacement over a wider range.

5. The combination of claim 2, characterized f in this that the locallyintroduced unmodulated carrier frequencyY is the kth harmonic of `the incoming signal carrier frequency.

4modulating element alocally introduced carrier A frequency which is the kth harmonic of the signal carrier frequency, and deriving from the output of the demodulatorthe signalwwave.

7. The combinationof claim 6 further characterized in this that the method includes so shaping the locallyintroduced carrier frequency capacitance and. a large resistance in series and means to derive the signal from the generated pulses.r y Y 9. In a receiving system for waves the frequency ofwhich has been modulated in accordance with signals, a. demodulation circuit comthat it is saw-toothed in form whereby theregion of linear demodulation is extended. A

8. In 'a receiving system for waves `the frequency of which has been modulated in accordance with signals, a demodulation circuit comprising a magnetic harmonic generator acting as a unitary self-limiting pulse generator, the generator including a non1ine`ar,saturating core magnetic coil on which the received frequency modulated wave is impressed, a load circuit shunting the non-linear coil comprising a large

Images